<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>css</title>
    <style>
        #box{    
            width: 300px;
            height: 300px;
            border: 25px solid green;
            padding: 50px;
            margin: 25px;
            background-color: red;
        }
    </style>
</head>
<body>
    <div id="box">这是内容</div>
</body>
</html>